The Ground
Zero here was in the original sense of word, a
nuclear blast site. The thermal energy may absorb heath
at a rate of 10 E 23 ergs / cm2 sec and near the bomb all
surfaces may heat to 4000 C or 7200 F igniting or
vapourizing violently. Source: US Department of Defense
& US Department of Energy, Glasstone  Dolan
The Effects of Nuclear Weapons (1980).

The
thermonuclear bomb used was a pure hydrogen
bomb, so no Uranium or Plutonium at all. The basic
nuclear reaction is Deuterium + Tritium > Alpha + n.
The ignition of this is the Fine part, either with
powerful beam array or antimatter (in a very certain way
to get the necessary directed-energy effect, in order not
to level the adjacent blocks of high-rise buildings,
too).